ASA 127th Meeting M.I.T. 1994 June 6-10

5aNS1. Passive-active isolator control of sound radiation from a
raft-cylinder system.

This paper analytically investigates the active-passive control of sound
radiation from an elastic finite cylinder containing a rigid raft mounted on
active-passive isolators. The disturbance is narrow band and acts at various
locations on the internal raft. The control cost function is constructed from
radiated far-field pressure or supersonic shell structural wave-number
estimates in contrast to the usual practice of minimizing vibration directly
under the mount attachment point. A control effort term is included in the cost
function to overcome the problem of an undetermined system when many isolators
are used. The results demonstrate that, using an acoustic-based cost function
in conjunction with a fully coupled control approach of the active-passive
isolators leads to significant radiated sound reduction with a very low control
effort. The mechanism is similar to ``modal restructuring'' observed in
previous plate studies. The results are contrasted to direct minimization of
vibration at the attachment points. The influence of passive variables such as
mount damping and stiffness on the control forces and the associated sound
reduction are also studied. [Work supported by ONR.]